1. Field of Invention
This invention relates to radiography. In a primary application the invention relates to the encoding of multiple x-ray images on a single storage phosphor screen detector.
2. Discussion of Prior Art
A widely used approach in x-ray imaging is to make multiple images of an object with changed conditions and then to process the image data to enhance the image or to extract more information. For example, in angiography, images are recorded before and after the injection of a radio-opaque contrast agent into the circulatory system. The images are then subtracted photographically to visualize only the blood vessels and eliminate the static anatomical features. Another example is energy selective radiography. Here, images are made using different effective x-ray energy spectra. The data from these images can be processed using the method described by U.S. Pat. No. 4,029,963 (1977) issued to R. E. Alvarez and A. Macovski. In accordance with this method, the data from two images are processed to calculate the photoelectric and Compton scattering components of the attenuation. These components, representing essentially atomic number and density, can be combined to represent different materials such as bone or soft tissue. These applications of multiple images all require spatially registered data, acquired in a short time, with good quantitative accuracy.
X-ray images are most commonly recorded on film. Although widely used, film has substantial problems for multiple image applications. It has excellent spatial resolution but poor quantitative capabilities. The quantitative response is critical because multiple image applications require processing of the image data instead of simply viewing it. With quantitative processing, such as subtraction, inaccuracies introduce errors in the final results. Film's quantitative response is highly nonlinear with a small dynamic range. Furthermore, the response depends critically on the development conditions and varies from film to film. Another problem is that multiple images must be recorded on different films. This makes it difficult to maintain spatial registration between the images on different films. It also requires a mechanical film changer which is made complex by several factors. First, the films are large, typically 35 cm, so it is difficult to move them rapidly. The changer must also use an intensifying screen. In medical radiographs, the x-ray photons do not expose the film directly. Instead, the x-rays are detected by the intensifying screen which produces visible light that, in turn, exposes the film as in a contact print. To avoid blurring the image, the film must be squeezed against the screen. Thus, the mechanical changer not only needs to move the films but to actuate a pressure to maintain screen/film contact. Because of all these factors, the time to change a film is long, one second or more. This is too slow to record rapidly changing structures such as the beating heart.
Some of these problems can be solved by encoding multiple images on a single film. This has the following advantages. Since the images are on one film, they are automatically spatially registered. The rate of image acquisition is multiplied by the number of images encoded. U.S. Pat. No. 4,413,353 (1983) "X-ray Encoding System Using an Optical Grating" issued to Macovski et al. describes an apparatus and method to effect this encoding. The method encodes multiple images by a technique analogous to amplitude modulation of radio waves. In accordance with the patent, the images are modulated by an optical grating placed between the intensifying screen and the film. The grating consists of alternating transparent and opaque stripes. The opaque stripes block visible light produced by the intensifying screen from reaching the film. The optical grating can be used to encode multiple images by making a first exposure, physically moving the grating by one half period, then making a second exposure. An alternative is to use an optical grating light valve whose alternate bars' light transmission can be switched off and on. Energy spectrum information can be encoded by changing the x-ray source spectrum between the exposures. The encoded images can be reconstructed by scanning the film and processing the resultant signal electronically.
The approach of U.S. Pat. No. 4,413,353 is limited to detectors with a separate light emitting intensifying screen and light recording medium. While this is the case with conventional film radiography, film has the quantitative problems mentioned above. The method also requires motion of the optical grating or a complex light valve. The grating must be moved precisely one half period and then pressure applied to maintain contact between the intensifying screen and the film. The light valve must be as large as the x-ray film and may contain thousands of elements.
An alternative to an intensifying screen/film detector with excellent quantitative properties is storage phosphor screens. X-ray imaging systems using these screens are described in U.S. Pat. No. 3,859,527 (1957) issued to G. W. Luckey. Storage phosphor screens function as a re-usable x-ray film. They are used in a cassette, similar to a film cassette, during medical examinations to acquire a an x-ray image. The image is stored as a latent image in the material of the screen. After the examination, the screens are removed from the cassette and scanned in a laser scanner. The scanner reads out the latent image from the screen and converts it to electronic digital signals. The digital signals are processed by a computer, then viewed on photographic film or on a cathode ray tube computer console. After the latent image is read out by the laser scanner, the screen can be erased and re-used. Storage phosphor screens have excellent quantitative properties. They have wide dynamic range (approximately 1000 to 1) and are linear and stable.
Storage phosphor screens, however, combine the functions of the intensifying screen and film in one screen so they can not be used with the approach of U.S. Pat. No. 4,413,353.